Ionization energy (IE) answers a simple but profound question: how hard is it to rip an electron away from an atom? Measured in kilojoules per mole (kJ/mol) or electron volts (eV), it quantifies the grip the nucleus has on the outermost electron. The first ionization energy removes the first electron; the second removes the next, and so on. Each successive removal requires more energy because you are pulling a negative charge away from an increasingly positive ion. For example, sodium's first IE is 496 kJ/mol, but its second IE leaps to 4,562 kJ/mol — nearly ten times higher — because that second electron comes from a stable, fully filled inner shell.
The periodic trends in ionization energy are nearly the mirror image of atomic radius trends. Moving across a period from left to right, IE generally increases because the growing nuclear charge pulls electrons closer and holds them more tightly. Lithium's first IE is 520 kJ/mol, while neon, at the end of the same period, reaches 2,081 kJ/mol. Moving down a group, IE decreases because the outermost electron is farther from the nucleus and shielded by more inner electron layers. Helium has the highest first ionization energy of any element at 2,372 kJ/mol, while francium has the lowest at about 380 kJ/mol. There are small dips in the trend — for instance, oxygen (1,314 kJ/mol) is slightly lower than nitrogen (1,402 kJ/mol) because of electron-electron repulsion in oxygen's doubly occupied 2p orbital.
Ionization energy has enormous practical importance. Elements with low IE (alkali metals, alkaline earths) lose electrons readily, which is why they are so reactive and form ionic compounds. Elements with high IE (noble gases, halogens) resist losing electrons, instead gaining or sharing them. The jump between successive ionization energies reveals core vs. valence electrons and confirms why elements form the charges they do — magnesium's huge jump occurs after removing two electrons, confirming its +2 ion. In plasma physics, ionization energies determine which elements ionize first in stars and fusion reactors.